For a B2B service, I use Auth0. It does cost $20 or so per month for the lowest tier, but when I need to enable enterprise login mechanisms for a single custome

For a B2B service, I use Auth0. It does cost $20 or so per month for the lowest tier, but when I need to enable enterprise login mechanisms for a single customer, it will be worth every penny to have that be a simple thing to enable versus a whole engineering project.